Standard passive optical access networks have a range of the order of 20 kilometers (km). This limited range is linked to the fact that in passive optical networks the optical components, for example couplers, multiplexers and optical fibers, cause signals in transit in the network to lose optical power, and the signals being transmitted cannot be amplified to compensate such losses without incurring constraints. In a passive optical network the downlink optical signals, i.e. the optical signals sent by the central office to users, and the uplink optical signals, i.e. the optical signals sent by user equipments to the optical central office, are carried by a single optical fiber. This reduces the cost of the network. However, using a single optical fiber to carry the uplink and downlink optical signals introduces constraints on the power at which those optical signals are transmitted, which leads to the limited range of the network.
Although the range of passive optical access networks is sufficient in urban areas, where users are at relatively short distances from the optical central offices, of the order of 5 km to 10 km, this does not apply to users in rural areas, where users are often geographically dispersed and are therefore usually situated at a distance from the optical central offices greater than the standard range of a passive optical network. Those users are therefore unable to benefit from the high transmission bit rates offered by passive optical networks and thus from the services on offer that require a high bit rate connection.
The inventors of the present patent application have previously constructed a long-reach passive optical network described in French patent application No. 06/52705 in the name of the same applicants as the present patent application. It is a point-to-multipoint network, for example, as shown in FIG. 1. An optical central office OC constitutes a first end of the network. A first end of an optical fiber 14 is connected to the output of the optical central office OC. A second end of the optical fiber 14 is connected to the input of an optical coupler 15 having one input and N outputs, N representing the number of branches in the network. The optical fiber 14 is referred to as the main branch of the network. A first end of an optical fiber 16j, jε{1, 2, . . . , N}, is connected to one of the N outputs Sj of the optical coupler 15. A second end of the optical fiber 16j is connected to a line termination device 17i, iε{1, 2, . . . , N} to which one or more users are connected. The optical fibers 161, to 16N are referred to as secondary branches of the network.
The optical central office OC includes a first laser 10 producing an optical signal associated with a particular wavelength. In the network this optical signal carries data addressed to users connected in accordance with the time division multiplexing principle. The optical central office also includes a second laser diode 110 producing an amplification optical signal associated with a particular wavelength different from the wavelength associated with the data optical signal.
In a network of that kind, a section 18 of erbium-doped optical fiber is inserted into the main optical fiber 14. The optical fiber section 18 serves as a passive amplification medium.
The amplification optical signal from the first laser diode 110 excites the erbium atoms in the optical fiber section 18. When the erbium atoms return to their non-excited state, they release photons in accordance with the stimulated emission principle, at a wavelength that corresponds to the wavelength of the data optical signal in transit in the network. These photons increase the optical power of the data signal. That technique is called remote amplification because the amplification medium 18 is in the network but the amplification means, here the laser diode 110, are in the optical central office OC. Thus by increasing the optical power of the data signal I, it is possible to make it travel a greater distance. That kind of network can achieve a range of the order of one hundred kilometers.
Other media and other in-line passive amplification techniques exist, of course, such as the Raman effect amplification technique, which uses the optical fiber 14 as an amplification medium.
However, although using passive amplification media reduces the costs of a passive optical network, passive optical access networks continue to be costly for telecommunications operators. The terminations of such passive optical networks are equipped with numerous costly components that consume electrical power, such as lasers producing the data signals, laser diodes serving as amplification means, and active transmission means disposed in the line termination devices. Moreover, such networks serve a large number of users, which increases the number of such components and therefore the cost of the network. There is therefore a need to reduce further the operating costs of such passive optical networks.